This invention relates to a circuit for scanning a keyboard and for actuating a display device, particularly a liquid crystal display device.
A type of visual display device coming into popular use incorporates so-called liquid crystals. Such devices are often used in conjunction with a keyboard and a microprocessor or microprocessor type device (such as a calculator chip). In the prior art, other types of display devices including arrays of light emitting diodes and vacuum flourescent devices have been used in conjunction with such microprocessor and microprocessor type devices. Further, in the prior art, it has been known to strobe the keyboard using lines which are also used for strobing an array of light emitting diodes, for instance. For example, a calculator utilizing a display drive and keyboard scan technique whereby a portion of the keyboard lines are interconnected with a portion of the display lines is disclosed in U.S. Pat. No. 3,984,816 for an "Expandable Function Electronic Calculator," which issued Oct. 5, 1976 and is assigned to the assignee of this invention. Such techniques were employed to reduce the number of pins on the calculator chip, to reduce the cost of packaging the calculator chip and to reduce the cost of manufacturing a calculator using the chip.
For various reasons, such prior art techniques have not been found to be applicable to devices utilizing Liquid Crystal Display (LCD) devices. For instance, the displays comprised of arrays of light emitting diodes can be operated on as little as a 1/15 duty cycle. In contradistinction, the lowest duty cycle currently available for a liquid crystal display having acceptable contrast levels (e.g. twisted nematic liquid crystal displays) is generally believed to be a 1/3 duty cycle. It should be evident to those skilled in the art that as the duty cycle of the device decreases, the number of pins required to interconnect the display device with a microprocessor or microprocessor type device increases. Further, because of the nature of the preferably Alternating Current (A.C.) signals applied to the segment lines and drive lines of a liquid crystal display device, this tends to complicate sensing techniques for sensing a key-down at the keyboard if either of these lines are used to drive the keyboard. As a consequence of the foregoing, prior art microprocessor type chips adapted for use with LCDs have been packaged in forty or forty-two pin packages. The display pin-out for such chips required on the order of twenty-eight pins, and the keyboard required on the order of ten pins plus battery connections and so forth resulted in such comparatively large pin-outs. On the other hand, calculator chips in twenty-eight pin packages are well known for displays utilizing light emitting diodes. It is often preferable, however, because of the lower current requirements of liquid crystal displays compared to light emitting diode displays, that liquid crystal displays be used in modern calculators to prolong the battery life of such calculators.
It was on object of this invention that a microprocessor type chip be packaged in a standard twenty-eight pin package and be interconnectable with a source of power, a keyboard, and a liquid crystal display.
It is another object of this invention that a calculator chip be interconnected with a matrix keyboard and a liquid crystal display device using a small number of interconnections.
It is still yet another object of this invention that a calculator chip be interconnected with a liquid crystal display device by a plurality of interconnections and that selected ones of said plurality also be used to interconnect a matrix keyboard with the calculator chip.
The foregoing objects are achieved as is now described. In a preferred embodiment of the invention, a calculator chip is interconnected with a liquid crystal display device via a plurality of conductors, the chip including display actuation circuits for providing normal four or five voltage level actuation signals to the LCD via these conductors. A matrix keyboard is provided which has two sets of conductors, these sets being coupled to the aforementioned plurality of conductors. Keyboard scanning signals or pulses are superimposed on the display actuation signals being supplied to those conductors coupled to the LCD and to one set of the keyboard conductors. These scanning signals or pulses preferably occur in time relation to signals occuring upon one of the display drive line conductors, preferably add no or little DC component to the display actuation signals and preferably have an RMS value small in comparison to the RMS value of the actuation signals. The display actuation circuits coupled to the other set of keyboard conductors are arranged to permit their outputs to float during the times the keyboard scanning signals or pulses are being supplied to the one set of keyboard conductors and are also coupled to sensing circuits for detecting such keyboard scanning signals or pulses, thereby indicating that a switch on the matrix keyboard has been closed. Based on the time a scanning signal or pulse is detected by the sensing circuit and depending upon which conductor of the other set of conductors a scanning signal or pulse is sensed, the particular switch closed on the matrix keyboard may be determined.
It should become evident that while this invention is preferably practiced with a liquid crystal display device, the basic technique herein disclosed may be used with other type display devices as well as liquid crystal displays.